Metals such as transition metals, noble and base metals are used as catalysts in many chemical reactions. Catalysts generally increase the rate of chemical reaction which results in higher production rate in industry. Some catalysts are also used to drive reactions along a desired path, i.e., the catalysts make formation of certain chemicals energetically favorable over other chemicals.
In general, catalysts are expensive, and hence it is necessary to utilize a given amount of catalyst to its maximum potential. This is done by maximizing the catalyst surface area, i.e., by increasing its dispersion. Catalysts can be used as solids or liquids. The solid catalysts are generally supported on high surface area supports. The properties of the support become very important in such cases.
Activated carbon has also been used as a support for metal catalysts, e.g., noble metals, because of its very high surface area and other properties such as inertness. Such catalysts are used (powder or beads form) in various chemical and petrochemical reactions. These catalysts are normally made by dispersing noble metal particles on preformed activated carbon (incipient wetness technique).
The incipient wetness technique involves dispersing the activated carbon powders in a solution of a metal salt. The activated carbon powder is then impregnated with the solution. The powder is dried, and heated to appropriate temperature to decompose the salt to the desired metal or metal oxide catalyst. Multiple impregnations are usually required to obtain the desired quantity of catalyst on the activated carbon. Surface properties of activated carbon powders play a very important part in the dispersion of the metal catalyst obtained. Oxygen content and surface pH of the carbon powder has to be carefully controlled to obtain a good dispersion of the metal on the activated carbon. The various steps that are involved in this process result in a very expensive activated carbon supported catalyst. Thus, it is important to maximize utilization of the catalyst. Although carbon supported catalysts are commercially utilized, performance improvements are always sought after.
More recently, a method for producing a highly uniform dispersed catalyst on activated carbon is U.S. Pat. No. 5,488,023. This method involves combining a carbon precursor and a catalyst precursor, followed by curing, carbonizing, and activating the carbon precursor to produce a continuous uninterrupted activated carbon. The activated carbon with dispersed catalyst can be in the form of a coating on a substrate, granules, or a shaped monolithic body.
Despite the advantages provided by this latter method of combining catalyst and carbon precursors, the types and amounts of catalyst precursors are limited because care must be taken that the carbon and catalyst precursors are compatible and that the carbon precursor solution is not overly diluted with the catalyst precursor.
Therefore it would be advantageous to have a method of making an activated carbon supported catalyst that is flexible enough to accommodate a wide variety of metal catalysts and that produces a uniformly distributed catalyst.
The present invention provides such a method.